Logical Reasoning

PT155 · S1 · Q23 On Earth, biological activity

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On Earth, biological activity leads to a change in the ratio of isotope S-34 to isotope S-32.

Conclusion (Therefore)

It is unlikely that life occurred on Mars.

Evidence

On Earth, life leads to a change in the ratio of isotope S-34 to S-32.

A new meteorite that came from Mars shows ratios of these elements found only in terrestrial minerals dating from before the beginning of life on Earth (i.e. the meteorite has the pre-life ratio of 34 to 32)

Evaluate

The author is thinking that if Mars had had life, then that life would have converted the S-34 to S-32 ratio. And then if a meteorite came from Mars to Earth, then it would be showing that post-life ratio.

Since the meteorite that came from Mars showed the pre-life ratio, the author is concluding that there was never life on Mars.

Given that ... this meteorite has the pre-life ratio we see on Earth

How could we still argue that ... life did occur on Mars?

Maybe on Earth biological life changes the ratio, but on Mars it doesn't, so even if there was life it would have still been the same ratio.

Maybe there was life on some areas of Mars and it changed the isotope ratio in those areas, but this meteorite came from a life-free part of Mars, so it shows the pre-life ratio.

Goal

Since this is an EXCEPT, there are going to be four answers that weaken. We aren't going to anticipate all four. We just want to set up our brains to interpret these answers. They fit into either of these sentences:

23.

Each of the following, if true, would weaken the argument except:

  1. Life forms that have a

    Weakens

    "Life did occur on Mars. The reason that this Martian meteorite only shows the pre-life ratio is because ...." .... life forms that have evolved on other planets, unlike those on Earth, don't change the ratio of S-34 to S-32.

    7% picked this

  2. The effects of life on

    Weakens

    "Life did occur on Mars. The reason that this Martian meteorite only shows the pre-life ratio is because ... " .... life forms don't necessarily change the ratio from S-34 to S-32; it's complicated; it's based on some other climate and environmental factors that are different on Earth vs. Mars. This is essentially the same as (A). We're able to argue that on Mars, life doesn't change the ratio of S-34 to S-32 because of different climate and environmental factors, so that's why even though there was life on Mars, the Martian meteorite won't show the ratio that happens on Earth when life comes on the scene.

    10% picked this

  3. Correct

    The ratio of S-34 to

    Why this is right

    One of our possible objections was that "There was life on some spots of Mars, and in those spots the ratio changed as it did on Earth. But ... this meteorite happened to come from a different spot of Mars, where life had not affected the ratio." This answer strengthens by ruling out that objection. It's saying that the ratio in the meteorite was fully representative of the ratio present on Mars when that meteorite left.

    Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.

    53% picked this

  4. Relatively few terrestrial mineral samples

    Weakens

    "Life did occur on Mars. The reason that this Martian meteorite shows a pre-life ratio is that ... .... life doesn't usually change the isotope ratio when it comes to terrestrial minerals. This answer is weird, and I may be misunderstanding its intended thrust, but since (C) actually strengthens the argument, it's a safe bet to pick (C) and assume this somehow weakens. It seems like this answer is saying, "even in present day samples of terrestrial minerals, you can scarcely find the post-life ratio of isotopes", which suggests that terrestrial mineral samples aren't a good way to check on whether life has / hasn't shifted the ratio. A meteorite is a big chunk of rock, and rocks are terrestrial mineral deposits. So it seems like the intended force of this answer is saying, "we shouldn't really expect a meteorite to show whether or not the ratio changed on Mars, because even on Earth, where we know life has occurred and where we know life has shifted the ratio, you can barely find evidence of that shifted ratio when you check mineral samples".

    16% picked this

  5. The current ratio of S-34

    Weakens

    "Life did occur on Mars. The reason that this Martian meteorite shows S-34 (the pre-life isotope on Earth) is because ...." .... this meteorite left before life started on Mars. If you went to Mars nowadays and checked out the ratio, you'd see it's the post-life ratio we see on Earth, so you'd think, "Wow, I guess Mars had life!" But since this meteorite left before life arrived, it still has that pre-life ratio that is a relic of the past.

    13% picked this

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